As is well known in the art, an AWD (All Wheel Drive) vehicle can be provided with at least one hydraulic disc coupling for distributing the driving torque from the vehicle engine to all of the vehicle wheels. Especially, such a coupling may be provided in the drive line between the front axle and the wheels of the rear axle, most often in the vicinity of the rear axle differential.
Sometimes, it may be desirable to use the AWD vehicle in a FWD (Forward Wheel Drive) mode. In this case the coupling is disconnected, i e its discs are separated from each other.
By operating the vehicle with the coupling in a disconnect mode, rotating masses in the vehicle are reduced, leading to a lower fuel consumption.
In a practical case a propeller shaft of the vehicle, possibly including one or two bevel gear transmissions, has a hydraulic disc coupling at one end and a simple coupling, for example a dog clutch, at the other end. In the disconnect mode both these couplings must be disconnected for obtaining the desired effect.
A pump actuator system is disclosed in WO 2011/043722. In such a system the supply of hydraulic oil to the coupling piston is governed by the rotative speed of a pump.
For obtaining the desired result in a disconnect mode, it may be of importance to reduce the drag torque within the disc package of the coupling by removing cooling and lubricating oil, that is supplied to the package in the connect mode.
A gravity lubrication system may be used. Such a system may comprise a side reservoir for cooling and lubricating oil, which is supplied by gravity to the disc package and returned to the side reservoir by forces created by rotating disc coupling parts.
A shut-off valve is provided to shut off the supply of oil in the disconnect mode and to reestablish the supply in the connect mode. The reestablishment is activated by hydraulic pressure controlling the coupling.
The use of a normal slide valve as the shut-off valve has certain disadvantages. These disadvantages also apply in a second use of a shut-off valve, namely in a normal AWD coupling system without any disconnect/connect function. In a stand-by mode of such a coupling it may be advantageous to reduce or eliminate drag losses due to oil splashes from rotating parts in the coupling.
A normal slide valve has a certain stroke and area. This means that a certain oil volume has to be branched-off to the shut-off valve from the control oil flow to the coupling, which negatively influences the coupling reaction time at the shift-over from disconnect to connect mode or from stand-by mode to torque transfer mode.
A certain hydraulic pressure must be reached to activate and move a normal slide valve against the forces from sealings and a return spring. In a slide valve the sealing is primarily used for preventing oil leakage at intermediate and high pressure ranges (where response times in the AWD mode can be influenced). The friction of the sealing necessitates a higher return spring force, which means that the minimum pressure rises for two reasons. A high minimum pressure level will restrict the lowest possible torque transfer of the coupling in a negative way.
Due to difficulties at the sealing manufacturing the diameter of the slide cannot be too small, as the friction then will be proportionally too high. This will be a disadvantage for the oil consumption mentioned above.